This invention relates to aeration systems for aerating wastewater streams which are subject to varying flow rates.
In certain wastewater treatment systems, a stream of wastewater is subjected to biological treatment in an aeration basin containing active sludge followed by clarification in a clarifier. In many cases, the rate of flow of the wastewater entering the basin is subject to large variations resulting, for example, from sudden storms producing a surge of rain water into the sewer system from which the wastewater stream is supplied to the aeration basin. In such cases, the surge of wastewater passing through the aeration basin may carry away much or all of the active sludge or "biomass" in the aeration basin and in the clarifier, causing a substantial delay in restoration of biomass to the system to permit normal operation.
To counteract this effect, the Stevens U.S. Pat. No. 3,549,521 discloses a two-stage treatment system with an aerator and a clarifier in each stage and having valves arranged to connect the two stages either in series or in parallel. As long as the incoming wastewater flow rate is below a predetermined level, all of the wastewater to be treated passes through both stages in series, but when the flow rate exceeds a selected level, a portion of the incoming wastewater is diverted directly to the second stage and the outlet from the first stage is diverted to the output from the system. In each stage sludge is returned from the clarifier to the input for that stage or is supplied to a digester. This arrangement, however, does not avoid the problem of loss of biomass from both treatment stages in the event of prolonged surges.
The Chipperfield et al. U.S. Pat. No. 3,403,095 discloses a wastewater treatment system having a first biological treatment filter supplying treated wastewater to a primary clarifier from which the effluent passes to a secondary treatment stage and then to a final clarifier during normal operation. When rainfall causes a surge resulting in a flow rate exceeding the capacity of the primary clarifier, a portion of the output from the first biological treatment stage is bypassed to the final clarifier, thereby eliminating the primary clarifier and secondary treatment for that portion and the portion which passes through the primary clarifier and the secondary treatment stage may bypass the final clarifier. This arrangement also does not avoid the problem of loss of active sludge from both stages during extended periods of excessive slow rates through the system.
U.S. Pat. No. 3,623,976 to Cessna discloses a wastewater treatment system which includes a primary clarifier, a biological filter and a secondary clarifier which are normally connected in series. In order to avoid loss of solids from the system during high loading, a bypass is provided to divert incoming liquids around the primary clarifier and the biological filter then functions as a trickling filter, which inhibits loss of active solids. This arrangement, however, requires a biological filter construction which can act as a trickling filter under such conditions.